Internal secular evolution complements environmental processes such as hierarchical clustering and harrassment in shaping galaxies. Thirty years ago, Hubble classification was in active use, but we also knew of a long list of commonly observed features in disk galaxies, including lens components, boxy bulges, nuclear bars, and central starbursts, and also a list of unique peculiar galaxies (e.g., Arp 1966) that were unexplained and not included in morphological classification schemes. Almost all these features and peculiar galaxies now have candidate explanations within one of two paradigms of galaxy evolution that originated in the late 1970s. The peculiar objects have turned out mostly to be interacting and merging galaxies. And many previously unexplained features of disk galaxies are fundamental to our understanding that galaxies evolve secularly long after the spectacular fireworks of galaxy mergers, starbursts, and their attendant nuclear activity have died down.
Acknowledgments
It is a pleasure to thank Ron Buta and Marcella Carollo for providing many of the images used in the figures. David Fisher kindly took the images of NGC 3945 with the McDonald Observatory 0.8 m telescope, and Tom Jarrett provided the star-removed 2MASS images of NGC 3945. This paper is based partly on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by AURA, Inc. under NASA contract NAS 5-26555. We also used the NASA/IPAC Extragalactic Database (NED), which is operated by JPL and Caltech under contract with NASA.